This invention relates to microwave oscillators comprising a waveguide source cavity in which there is mounted an oscillator device, particularly but not exclusively a Gunn diode, for generating power of a desired microwave frequency; the invention is concerned with stabilizing the oscillator frequency. Such an oscillator may be designed for use as the transmitting source in microwave transmission equipment (e.g. in a video distribution system) or in radar for example, or it may be designed for use in a non-transmitting mode (e.g. for a local oscillator in instrumentation). The invention also provides a simple microwave transmitter comprising such an oscillator as its microwave source.
For many waveguide sources operating within the frequency range of 26 GHz to 100 GHz, there is a need for low-cost control arrangements to improve the long-term behaviour of the source, particularly its frequency stability and/or its amplitude stability. Thus it is found that over a 2 year period the centre frequency of the source may drift by 10 MHz or more.
It is known to monitor the output of a microwave oscillator via a directional coupler mounted outside the oscillator cavity. The directional coupler is designed so as to sample the waveguide output at the oscillator frequency and requires an appropriate mounting outside the oscillator cavity. Especially with oscillators in the mm-wave region of the spectrum, such couplers can be expensive to manufacture and to mount. Furthermore the directional coupler has a finite fixed insertion loss which may be considerably more than the sample power coupled out at small coupling values. For mm-wave couplers this fixed loss typically may be about 0.5 dB and so the available power from the source may be reduced by as much as 10%.
Such losses can be avoided by adopting a different approach to frequency stabilization, using a cavity resonator coupled to the source cavity. In an example of this known different form as proposed in published Japanese patent application kokai JP-A-55-83304, the cavity resonator also forms a detector section of the oscillator. Thus, the microwave oscillator of JP-A-55-83304 comprises a waveguide forming a source cavity separated from a detector section (the cavity resonator) by a reflective wall, and an oscillator device mounted in the source cavity for generating power of a desired microwave frequency; the wall is present at one end of the source cavity for reflecting the microwave power generated by the oscillator device; a first opening is present in another wall portion of the source cavity to provide a primary output of the microwave power; a coupling aperture is present in the reflective wall to provide as a secondary output a sample of the microwave power from the source cavity to the detector section; detector means is associated with the detector section for generating from the sample a signal indicative of whether the desired microwave frequency is being generated in the source cavity; a bias terminal to the source cavity serves for applying a bias signal for controlling the microwave power generated in the source cavity.
In this proposed oscillator of JP-A-55-83304, the detector means is a detection diode outside the cavity resonator but connected thereto by a coupling part projecting into the cavity resonator. The cavity resonator amplifies the extracted sample at the resonant frequency. A load resistance is connected to the detection diode, and the voltage developed across this load resistance is used to monitor deviation of the oscillator frequency from the resonant frequency of the cavity resonator. No voltage is developed across the resistance when the oscillator is generating an unwanted wavelength differing from resonance; when this condition is observed the bias signal to the oscillator is reduced to suppress the unwanted wavelength by stopping the oscillation.
Although the provision of such a detection arrangement with the cavity resonator indicates when the generated frequency deviates from the resonance, it does not indicate whether the deviation is to a higher or lower frequency, and the circuit arrangement of the detection diode and load resistance does not make provision for automatically regulating the bias signal to the oscillator so as to correct its frequency operation. Furthermore, such a cavity resonator can be expensive to manufacture.